Chemical sensor arrays using integrated photonics have attracted significant attention because of their potential for large area environmental monitoring and high throughput screening for biomedical discovery. Advanced technologies using absorbance, surface plasmon resonance (SPR), and fluorescence detection have been developed to realize chip-scale optical sensors. For instance, chemical sensors using micro-ring resonators with ppm-level detectivity have been demonstrated. SPR sensors using a perfect absorber or highly-doped semiconductors are utilized for multispectral infrared (IR) spectroscopy and gas identification.
Unfortunately, chip-scale mid-IR sensors have yet to be realized in silicon-integrated photonics. Silicon-integrated photonics are based mainly on conventional silicon-on-insulator (SOI) technology, in which a thin layer of silicon dioxide serves as an undercladding between the top crystalline silicon waveguide and the bottom crystalline silicon substrate to prevent light leakage through the substrate. Though SOI is mature and suitable for near infrared photonic circuits, it cannot be easily adopted for planar mid-infrared (mid-IR; e.g., λ=3 μm to 8 μm) devices since silicon dioxide becomes optically lossy at wavelengths greater than about 3.6 μm. Hence, conventional SOI devices are generally unsuitable for detecting absorption in the mid-IR portion of the electromagnetic spectrum.
Replacing the silicon dioxide layer with sapphire to create a silicon-on-sapphire (SOS) increases the transparency range beyond 3.6 μm, but may preclude chemical detection of double-bond functional groups, such as C═O, C═N, and C═C, with vibrational absorption between λ=5 μm and λ=8 μm, due to absorption by sapphire above λ=5 μm. Another platform using silicon on porous silicon was proposed for mid-IR devices, in which the waveguide cladding is a porous silicon layer created by high-energy proton beam irradiation and electrochemical etching. Though low-index silicon is obtained as the undercladding, the damage and scattering loss caused by high-energy proton beam irradiation has not yet been investigated.